This invention relates generally to gas analysis apparatus and methodology, and more specifically relates to an improved circuit for linearizing the response of an electron capture detector of the type utilized in gas chromatography systems or the like.
Within recent years, electron capture detectors have come into use in certain gas analysis environments. Such detectors are particularly applicable to gas chromatography systems -- where their sensitivity enables detection of very low levels of electron-capturing components.
An electron capture detector includes a pair of spaced electrodes and a source of ionizing radiation as, for example, a short range beta source such as tritium contained in a quantity of titanium tritide. An electric field is established across the electrodes to produce a current of electrons migrating from an irradiated zone between the electrodes. In the absence of a gaseous compound which can capture the electrons, (e.g., in the case where only the pure carrier gas from a chromatographic column is being provided to the cell), a given cell current is produced. When an electron-capturing substance enters the cell, however, along with the carrier gas, some of the electrons are captured -- with a resultant reduction in the number of free electrons and a consequent decline in the cell current. The resultant change in the current is thus an indication of the presence of the electron-capturing substance sought to be detected. By means of suitable circuitry changes in the cell current may be converted to values indicative of changes in the concentration of the electron-capturing substance.
In relatively early versions of the aforementioned electron capture detectors (ECD), DC techniques were utilized to apply a fixed potential difference between the electrodes of the detector in order to enable the aforementioned cell current. It was found, however, that such DC techniques tended to yield an unacceptably non-linear response. It was therefore proposed by a number of investigators to utilize instead a pulse sampling technique, pursuant to which the electrodes are polarized by a succession of short pulses. The pulse sampling technique was found to indeed improve the linearity characteristics of the detectors. Thus, for example, Lovelock, et al., in U.S. Pat. No. 3,634,754, disclosed a method for linearizing the response of an ECD intended primarily for use in gas chromatography applications. According to such method, a pulsed voltage is supplied at variable frequency to an ECD, the detector current is sensed, and the voltage pulse frequency is varied to maintain the detector current constant. The resultant pulse frequency is then taken as an indication of the concentration of electron-absorbing compounds within the detector cell.
Improvements in the methodology of Lovelock are set forth in Marshall, et al., U.S. Pat. No. 3,671,740. In the Marshall, et al. circuitry, an electrometer-amplifier is coupled to the detector to provide a first output signal representative of the relative concentration of electron-capturing compounds within the cell. A ramp generator, including a resettable oscillator which exhibits a linear change of voltage with time, provides a second output signal of continuously varying magnitude. A signal comparator, which is coupled to the electrometer-amplifier and to the ramp generator, provides an output pulse when the output signals of the electrometer-amplifier and the ramp generator are equal. A pulse generator coupled to the comparator generates a train of pulses in response to triggering by the comparator output. Successive such pulses reset the the ramp generator, and clear the electron-capture detector cell of free electrons. A frequency-to-voltage converting means coupled to the pulse generator ouput provides an output voltage signal proportional to the frequency of the pulse train.
A key aspect of the Marshall, et al. circuitry is the means for comparing the output from the ramp generator with the output from the electrometer-amplifier. The output of this ramp generator consists of voltage V, which is an inverse saw-tooth wave created by the discharge of a capacitor C. In operation, the capacitor is initially charged to the potential V.sub.r of a fixed reference voltage. The capacitor is then connected to a current source I so that the capacitor discharges at a constant rate. The output of the ramp generator at any time t is V = V.sub.r - It/C When V falls to a value equal to the output voltage of the electrometer-amplifier, the comparator causes the pulse generator to operate and the ramp capacitor is recharged to the potential V.sub.r.
The fixed values I and C in the Marshall, et al. circuitry fix a maximum time required for the capacitor voltage V to decay completely to zero. This maximum decay time in turn establishes the lowest allowed frequency f.sub.o of operation of the pulse generator. Electronically, it is not possible with this arrangement for the pulse generator to operate at frequencies below f.sub.o. This is a significant disadvantage where the circuitry is connected to an electron capture detector operating in conjunction with a gas chromatograph. The reason for this is that it is quite common for elevated temperatures within a gas chromatograph to cause "bleed" of contaminant gases into the carrier gas. Furthermore, it is common for the magnitude of this contaminant bleed change with time as the gas chromatograph is used to analyze samples. The bleed rate may thus increase or decrease. In either case, the electron capture detector and frequency modulated pulsed circuitry will commonly respond with a varying output signal baseline corresponding to either increasing or decreasing pulse frequency.
Where circuitry of the type set forth in Marshall, et al. is utilized, it is common practice in the prior art to manually adjust the current input to the electrometer-amplifier such that the pulse generator operates at or near its lowest frequency f.sub.o when carrier gas only is passing through the ECD, i.e., when there are no electronegative sample peaks. Because of varying bleed rates as mentioned above, it is not uncommon for the carrier gas environment within the ECD to change with time, sometimes in a direction such as to demand a pulse frequency even lower than f.sub.o. However, the output signal of the circuitry is electronically prohibited from decreasing below a value corresponding to f.sub.o, so that this output signal will no longer provide an accurate representation of the ionization environment actually existing within the ECD.